To protect passengers in a motor vehicle, the vehicles are increasingly equipped with production passive safety systems, in particular with airbags. Depending on the type and severity of the specific impact, the airbags are to be triggered in such a way that the particular passenger of the vehicle strikes the inflated airbag with sufficiently gentle force and is decelerated. It is usual, for example, to ignite an airbag in two stages and to control the time interval between the two ignition procedures. The control of the triggering classically occurs when the abrupt braking of the vehicle upon impact against an obstacle exceeds a threshold value, this control being implemented with the aid of an acceleration sensor, which is affixed on the rigid support structure of the vehicle and assigned to the control unit. Avoiding erroneous activation becomes increasingly more important, especially at low driving speeds. For reasons of product liability, but also because of testing regulations mandated by law for type approval, the impact velocity, in particular at low driving speeds, must be detected as accurately as possible, at least with respect to the magnitude, so that low speeds that are in close proximity to each other are also able to be discriminated. This is not possible with speed sensors commonly used in motor vehicles, at least in those cases where the conventional measuring tolerances of ∓10% are insufficient to reliably implement the discrimination, especially since additional variation parameters such as vehicle aging and the like must be taken into account.
Therefore, it is an object of the present invention to indicate a possibility of ascertaining a utilizable measure for the impact velocity, in particular at low vehicle speeds.
According to the present invention, what is achieved is the recognition that a certain time interval elapses between the first impact against an obstacle and the deformation onset of the rigid parts of the vehicle structure during which deformable parts of the vehicle structure are deformed. This time characteristic of the impact velocity is essentially proportional and may thus be utilized as a measure for the impact velocity. While this is true only for barriers that are identical in rigidity and mass, this happens to be the case in test regulations. It is particularly important to detect the impact onset, for example with the aid of an impact switch, and to detect the transition from light to heavy acceleration or delay. In particular the latter instant may be established by analyzing the integral values of acceleration signals.
This makes it possible at low vehicle speeds to distinguish even among vehicle speeds that
are in close proximity to each other and thus to comply with the US requirement according to NHTSA 208, for example, in the low-risk deployment range. The corresponding requirement, namely to differentiate between a frontal impact against a rigid barrier as the obstacle at 26 km/h and at 32 km/h may thus be met. On the same basis, it is also possible to classify the degree of overlap, in particular in order to ascertain whether the given overlap amounts to 100%, in accordance with the aforementioned regulation. Furthermore, in combination with other criteria, a conclusion may be reached on the same basis as to whether or not an impact against a hard barrier has occurred.
The obtained data may then be utilized in a variety of algorithms to influence the triggering conditions, possibly together with additional criteria.